Thanks for asking me to respond.
Pain in the calves are most commonly caused by venous congestion and pooling of unoxygenated blood. This usually follows working for long hours in the standing or sitting position. In addition weak valves in the veins impair return of blood and compound the situation. Sometimes there can be spasms in the calf muscles leading to what we call a cramps. Cramps happen due to dehydration and decrease in electrolyte levels, often die to poor intake of water. Rarely is it due to clots in the deep veins of the calves. This is usually accompanied by intense swelling and redness of the calves.
My suggestion for your mother is that she drink plenty of water, and keep her legs elevated. If the pain persists after a week, visit an ortho surgeon who can examine her and prescribe her meds if necessary. I cannot prescribe meds as Practo prohibits me from doing so.